The present invention relates to implantable bone prostheses, and more particularly to bone prostheses which are structural in the sense that they are formed of a strong material which is attached to a bone in a way to replace or reinforce all or a portion of the natural bone. Examples of such prostheses include bone plates which are fastened to the bone at a fracture site to connect separated pieces and provide a structural link across a break or crack. Such bone prostheses also include entire joints or articulations or portions thereof, such as are customarily employed to replace or rebuild weakened, diseased or damaged hip joints, knee joints or the like.
In general, it has been found desirable that the prosthesis become incorporated in the existing bone, or at least that new bone growth attach to surfaces of the prosthesis to form a strong junction therewith. Historically, early prostheses sought only a certain level of biocompatability or nontoxicity for the material employed in the body or outer surface of structural elements forming the prosthesis. This approach essentially treated prosthetic patches as though they were braces or struts used in building construction, and relied extensively on material strength and the fitting or the contour of the article to replace corresponding bone strength. Subsequently, it was learned that bone adheres better to textured surfaces and that particular shapes and sizes of the surface texture or relief on a prosthetic device enhance the regrowth of bone in and around the surface, and increase the strength of the junction so formed. Particular material such as calcined and sintered preparations of coral or mineral-like materials, such as calcium phosphate-hereafter referred to simply as hydroxyapatite (HA)--were found to especially enhance new bone growth and promote incorporation into active bone processes.
Taking for example, a simple prosthetic device such as a replacement stem of a hip joint, these insights as to bone growth enhancement have been applied in several ways, as follows. Such an assembly normally includes a ball, and a stem to which the ball is attached, wherein the stem is tapered, and possibly splined or otherwise shaped to fit within the femur and to rigidly anchor the ball and stem assembly to this major leg bone. The ball itself is generally a polished metal or metal/ceramic article, which may be permanently or removably affixed to the stem portion by a very strong and precisely machined or molded fitting, such as a tapered bore and post. The stem, on the other hand, is typically substantially all metal, such as a titanium or a cobalt chrome alloy, and is intended to bear the load and bending stresses transmitted between stem and bone along the upper leg. While historically femoral stems were initially simply cemented in position with an acrylic or similar cement, subsequent knowledge of bone growth has lead to the development of such stems having textured regions configured for promoting bone growth and enhanced gripping strength with the newly-grown bone. There has been a concomitant reduction and even elimination of the use of cement for initially attaching the stem to the bone at this site.
A number of ways have evolved for creating the aforementioned surface textures, including plasma spraying of metal droplets, baking-on or welding of thin wires or grains onto regions of the surface, and more recently, the direct casting of the article with regions of surface texture formed by a pattern in the casting mold. In the latter approach, several difficulties were initially presented. First, a casting procedure necessarily involves forming a suitable mold and, subsequent to casting of the metal article, removing the mold. The existence of surface texture on the mold and the cast part generally increases the difficulty of separating the two parts. Second, often desirable textures quite simply cannot be separated without breaking the mold. This feature has also presented difficulties for example in attempts to mass produce appropriate molds by conventional processes such as slip casting the mold body over a wax perform.
Problems of this type can be largely overcome by initially making the molds in an automated manner using techniques such as three dimensional printing, which are described in greater detail in commonly assigned U.S. patent applications Ser. No. 08/198,874 and the file wrapper continuation of that application filed on Jun. 6, 1995, as well as the techniques described in U.S. patent application Ser. No. 08/198,607 filed Feb. 18, 1994, as well as in U.S. Pat. No. 5,204,055 of Sachs et al., the disclosures of all of which are hereby incorporated herein by reference. Those applications taken together describe techniques for three-dimensional printing or building-up and curing of a form or a mold having an arbitrary bounding surface, so that the article cast therein may be formed with complex or arbitrarily-designed surface protrusions or indentations, including protruding walls of an undercut or overhanging type which would normally be not manufacturable in multiple copies, or which would present great difficulty of proper filling, finishing, or separation from the mold if so manufactured. It also allows the computerized storage and generation of shapes, and the precise modification or scaling of dimensions and contours to allow one to quickly identify and subsequently define shapes suitable for optimal bone growth and attachment, and to manufacture such textured prosthesis in multiple sizes.
So far as relevant to present application, mold and prosthesis forming techniques of the foregoing patent applications are assumed known and their teachings are incorporated by reference herein without further discussion. They allow one to cast a prosthesis with a desired shape of growth-compatible and grip-enhancing surface relief.
In addition to surface texture, various prior art techniques of forming a prosthesis use a bone growth material such as hydroxyapatite, either in the structural body or in regions of the surface of the prosthesis. One such technique has been to form a porous body of hydroxyapatite (HA), which is then filled by casting a metal for reinforcement into the porous body. Another approach is to apply the hydroxyapatite as a surface coating to an already-cast article. Such a coating may be applied, for example, by a plasma spray deposition process in order to firmly adhere the HA material to the surface of an already-cast article. Other techniques of applying an HA coating include painting-on a slurry of hydroxyapatite and baking it at a sufficiently high temperature.
However, the approach of applying such a coating to enhance bone growth and attachment at the surface of a prosthesis has generally fallen into disfavor because both the process of initial handling and implantation, and the resorbability and disintegration in vivo of the hydroxyapatite, result in shedding of flakes and microparticles, and these shed particles can cause adverse bodily reactions, such as lysis and cell reactions, or can become trapped or embedded as abrasive bodies in components in the joint space.
Accordingly, it would desirable to provide a bone prosthesis with enhanced bone growth properties but without the structural disadvantages of prior growth enhancement constructions.